When utilizing a SAW filter, the output of the SAW filter is typically coupled through at least one gain amplifier whose output is coupled back to the input of the SAW filter. The purpose of the gain amplifier is to ensure an overall loop gain greater than one, and to ensure that the output of the SAW filter provides an oscillatory signal. In general, if the output of the SAW filter does not provide an oscillatory signal, the differential inputs (or outputs) of the gain amplifier can simply be reversed thereby yielding an effective 180.degree. phase shift and ensuring that an oscillatory signal appears at the output of the SAW filter as is known.
In the past, prior art has simply resorted to manually inverting the connection from the SAW filter to the differential gain amplifier to achieve the proper phase for oscillation. However, this involves making new bond contacts either from the output of the gain amplifier back to the input of the SAW filter, or from the output of the SAW filter to the input of the gain amplifier. Moreover, this procedure typically results in having to do a re-layout of the board.
Hence, there exists a need for an improved circuit and method for providing a 180.degree. phase shift in a gain amplifier.